Remove AOT conditional exclusions for SignalR and delete deprecated methods

[JusterZhu]

Summary

SignalR Client supports Native AOT (confirmed by dotnet/aspnetcore#59133). The requirement is using JSON protocol + registering JsonSerializerContext — which UpgradeHubService already does. Therefore all AOT conditional exclusions for SignalR/Hubs/SilentPoll are unnecessary and should be removed.

Also removes deprecated abstract methods in AbstractBootstrap that are never invoked.

Changes

csproj

• Remove #define AOT constant
• Remove Condition="'$(PublishAot)' != 'true'" from 3 SignalR.Client PackageReference entries
• Remove 3 <Compile Remove> entries for AOT exclusion (Hubs/*.cs, HubDownloadSource.cs, SilentPollOrchestrator.cs)

GeneralUpdateBootstrap.cs

• Remove #if !AOT / #endif around HubDownloadSource initialization
• Remove #if !AOT / #else / #endif around LaunchSilentAsync method body
• Delete 3 deprecated override methods (StrategyFactory, ExecuteStrategyAsync, ExecuteStrategy) that throw NotImplementedException

AbstractBootstrap.cs

• Delete 3 deprecated abstract methods (ExecuteStrategy, ExecuteStrategyAsync, StrategyFactory) — only LaunchAsync remains as the sole abstract method